This invention relates to a method of making a foundation and is specifically concerned with the making of a foundation for a wind turbine.
A currently accepted method of making a foundation for a wind turbine is described in U.S. Pat. No. 5,586,417, to which reference should be made. The foundation is constructed of cementitious material poured in situ between inner and outer cylindrical corrugated metal pipe shells. The foundation is formed within a ground pit which is externally and internally back-filled.
An anchor ring is embedded in the lower end of the foundation and sets of inner and outer circumferentially spaced bolts have their lower ends attached to the anchor ring and their upper ends projecting outwardly of the top of the foundation. The upper ends of the bolts pass through holes in a base flange of a tubular tower resting on the foundation and nuts are threaded downwardly upon the upper ends of the bolts and against the base flange.
An essential feature of this foundation is that the bolts are all pre-stressed. This type of foundation is accordingly often referred to as a xe2x80x9ctensionlessxe2x80x9d tube.
It is an object of the present invention to provide a method of making a foundation, particularly a foundation for a wind turbine, which is more economical than the method of U.S. Pat. No. 5,586,417.
According to the present invention there is provided a method of making a foundation, particularly a foundation for a wind turbine, said method comprising:
a) forming a pit,
b) providing a cylindrical open-bottomed steel can,
c) lowering the can into the pit,
d) accurately aligning the steel can,
e) concreting the steel can in position in the pit, and
f) back-filling the interior of the can.
Concreting of the can in position in the pit is preferably effected in a first stage and a second stage, the first stage involving the introduction of concrete into the bottom of the pit to a depth sufficient to form a concrete layer in the open lower end of the can and around the lower end of the can, and the second stage involving the subsequent introduction of concrete around the can to substantially ground level. The second stage will preferably be carried out at least twenty four hours after the first stage to allow time for setting of the concrete introduced in the first stage.
The open-ended steel can preferably includes a plurality of sections of different wall thickness, with the lowermost section being of the lowest wall thickness and with the upper section of greatest wall thickness. The steel can may, for example, comprise three sections of different wall thickness. For a steel can having a depth of eight metres, there may be an upper section having a depth of 2 metres and a wall thickness of 23 mm., an intermediate section having a depth of 3 metres and a wall thickness of 18 mm., and a lowermost section having a depth of 3 metres and a wall thickness of 16 mm. It will be appreciated that the depths of the sections and the wall thickness can be varied depending on the required parameters for the finished foundation.
The pit is preferably formed by excavation and is preferably of substantially square cross-section. However, as opposed to excavating a pit of substantially square cross-section using a conventional excavating machine, it is possible to drill a circular hole, of greater diameter than the external diameter of the steel can.
Back-filling of the interior of the can may be effected using the as-dug excavated material. Compaction of the as-dug excavated material will not normally be required. Once the interior of the can has been filled to the required level, a layer of concrete is preferably laid on top of the in-fill material.
The can is preferably supported, during lowering thereof, from a three-point levelling support frame.
The upper end of the steel can is provided with fixing means designed to facilitate connection of the structure being supported by the foundation to the steel can. Such fixing means may comprise, for example, an inwardly extending flange at the upper end of the steel can, which flange is formed with a plurality of angularly spaced apertures to receive fixing bolts for connection of the flange at the upper end of the steel can to a corresponding flange at the lower end of the structure.
Other connection means may, of course, be provided, particularly if the structure is a wind turbine. For example, the upper end of the steel can may, when installed, extend above ground level to an extent such as to facilitate welding of the upper end of the steel can to the base of the wind turbine.
The foundation will thus function as a monopile and, in a typical example, the steel can will have a depth of 8 metres and an outside diameter of 3 metres. The pit which is formed to receive the steel can will then be of square configuration with minimum dimensions of 3.3xc3x973.3 metres square and 7.85 metres deep and maximum dimensions of 4xc3x974 metres square and 8 metres deep.